BIO 342Comparative Vertebrate AnatomyLecture Notes 11 - Nervous System

The Vertebrate Nervous System:

1 - receives stimuli from receptors & transmits information to effectors that respond to stimulation

2 - regulates behavior by integrating incoming sensory information with stored information (the results ofpast experience) & translating that into action by way of effectors

3 - includes billions of nerve cells (or neurons), each of which establishes thousands of contacts withother nerve cells

4 - also includes neuroglia cells that support, nourish, & insulate neurons

Subdivisions of the Vertebrate Nervous System:

1 - Central Nervous System - including the brain & spinal cord

2 - Peripheral Nervous System - including cranial nerves, spinal nerves, & all branches of cranial & spinalnerves

Source: http://www.david.curtis.care4free.net/neurons.htm

Neurons (or nerve cells):

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respond to stimuli & conduct impulses3 types - all with cell body & processes (axons & dendrites):

multipolarbipolarunipolar

Multipolar neuron

Bipolar neuron Unipolar neuron

Nerves = bundles of nerve cell processes; may be sensory, motor, or mixed

Spinal cord:

located in vertebral canalanatomical beginning is the foramen magnum of the skulllength varies among vertebrates:

in vertebrates with abundant tail musculature, the spinal cord extends to the caudal end of thevertebral columnin vertebrates without tails or without much tail musculature, the spinal cord extends to aboutthe lumbar region of the vertebral column

a cross-section of the spinal cord reveals gray matter & white matter. The gray matter consists ofnerve cell bodies, while the white matter consists of nerve cell processes (axons). These processesmake up ascending (sensory) and descending (motor) fiber tracts.

Used with permission of G. Mandl

Spinal nerves:

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arise from spinal cord by dorsal & ventral roots. The dorsal root exhibits a ganglion & is sensory,while the ventral root has no ganglion & is motor.early vertebrates:

dorsal & ventral roots did not unitedorsal roots were mixed (contained both sensory & motor fibers)no dorsal root ganglion

Rami - 2 branches of each spinal nerve:dorsal ramus - supplies epaxial muscles & skin of the dorsal part of the bodyventral ramus - supplies hypaxial muscles & skin of the side & ventral part of the body

Functional types of neurons in spinal nerves (& other nerves):somatic afferent - sensory from general cutaneous receptors (in the skin) & proprioceptors (inskeletal muscles, tendons, & joints)somatic efferent - motor to skeletal musclesvisceral afferent - sensory from receptors in the viscera (smooth muscle, cardiac muscle, &glands)visceral efferent - motor to smooth muscle, cardiac muscle, & glands

Brain:

the anterior end of the embryonic central nervous system exhibits 3 primary sections:prosencephalon (forebrain) - subsequently divides into the telencephalon (cerebrum) &diencephalon (epithalamus, thalamus, & hypothalamus)mesencephalon (midbrain) - develops without further subdivision & forms the tectumrhombencephalon (hindbrain) - subdivides into the metencephalon (pons & cerebellum) andmyelencephalon (medulla oblongata)

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Source: http://brainmuseum.org/development/index.html

Phylogenetic trend in vertebrate brains is for enlargement of forebrain:increasingly complex behaviors & muscle control:

coordination of limb movements more complicated (e.g., bipedal dinosaurs & birds)increased input of sensory information & increased output of motor responses

Source: http://www.colorado.edu/epob/epob3730rlynch/image/figure5-1.jpg

Myelencephalon - consists of the medulla oblongata & its major functions include:

origin of cranial nerves (VII - X or VII - XII)pathway for ascending & descending fiber tractscontains centers important in regulating respiration, heartbeat, & intestinal motility

Metencephalon - consists of the pons & cerebellum:

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Pons - pathway for ascending & descending fiber tracts & origin of cranial nerves V, VI, & VIICerebellum - modifies & monitors motor output:

important in maintaining equilibriumcoordinates & refines motor action

Mesencephalon - consists of the tectum which includes the optic lobes & auditory lobes:

optic lobes - receive fibers from retina; vary in size with relative importance of visionauditory lobes - receive fibers from inner ear

Diencephalon - consists of the epithalamus, hypothalamus, & thalamus:

epithalamus - includes pineal gland (epiphysis) that affects skin pigmentation (by acting onmelanocytes) in lower vertebrates & plays a role in regulating biological rhythms in higher vertebrateshypothalamus - regulates body temperature, water balance, appetite, blood pressure, sexual behavior,& some aspects of emotional behaviorthalamus - major coordinating, or relay, center for sensory impulses from all parts of the body

Source: http://www.colorado.edu/epob/epob3730rlynch/image/figure5-4.jpg

Telencephalon - consists of the cerebrum which, in turn, consists of 2 cerebral hemispheres

cerebrum has 2 regions: a dorsal PALLIUM (with medial, dorsal, & lateral divisions) & a ventralSUBPALLIUM (consisting of a striatum & a septum)all vertebrates have a cerebrum based on the same basic plan; major phylogenetic changes are dueto loss, fusion, or enlargement of the various regions.

medial pallium receives olfactory informationdorsal & lateral pallia receive other sensory input (including visual & auditory informationrelayed from the thalamus)

agnathans, fish, & amphibians - pallia are similar

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Source: http://www.auburn.edu/academic/classes/zy/0301/Topic19/Topic19.html

reptiles - pallium has 3 main divisions (medial, dorsal, & lateral) but also has a large DORSALVENTRICULAR RIDGE (DVR), derived from lateral pallium; DVR may be higher association areabirds - DVR expands further; dorsal part increases in size & is called the WULST; as in reptiles, theDVR appears to serve as a higher association area

Source: http://www.pigeon.psy.tufts.edu/avc/husband/avc2amnt.htm

mammals - do not have enlarged DVR but DORSAL PALLIUM is enlarged & is called theCEREBRAL CORTEX; cortex receives & analyzes sensory information & initiates motor activity

subpallium:septum - important part of the limbic system (regulates emotions & plays vital role in short-termmemory)striatum - also called basal ganglia; present in all vertebrates & controls sequence of actions incomplex movements

Cranial nerves - agnathans, most fish, & living amphibians have 10 cranial nerves; crossopterygians &amniotes have 12:

Olfactory nerve (I) - sensory nerve; sense of smellOptic nerve (II) - sensory nerve; sense of visionOculomotor nerve (III) - motor nerve to extrinsic eye musclesTrochlear nerve (IV) - motor to extrinsic eye musclesTrigeminal (V) - mixed nerve; sensory from skin of head & mouth (including teeth) & motor tomuscles of 1st pharyngeal arch (muscles of jaw)Abducens (VI) - motor to extrinsic eyeball musclesFacial (VII) - mixed nerve; sensory from lateral line of head, ampullae of Lorenzini, & taste buds;motor to muscles of hyoid archAuditory (VIII) - sensory from inner ear (balance & hearing)

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Glossopharyngeal (IX) - mixed nerve; sensory from taste buds & lateral line; motor to muscles of 3rdarchVagus (X) - mixed nerve; sensory from & motor to heart, anterior digestive system, mouth, gillpouches 2 - 5, & lateral lineAccessory nerve (XI) - motor to derivatives of cucullaris muscle (cleidomastoid, sternomastoid, &trapezius)Hypoglossal nerve (XII) - motor to hyoid & tongue muscles

Possibly useful mnemonics to aid in memorization of cranial nerves: On Old Olympus Towering Top A FinnAnd German Viewed A Hop or Oh, Once One Takes The Anatomy Final, A Good Vacation AppearsHeavenly.

Sensory Organs

Sensory receptors:

monitor the external & internal environment by responding to selected stimuli, then translating thosestimuli into nerve impulsesTypes of sensory organs:

somatic sensory organs - provide information about the external environmentvisceral sensory organs - provide information about the organism's internal environmentgeneral sensory organs - widely distributed over the surface & interior of the bodyspecial sensory organs - confined to the head (amniotes & terrestrial amphibians)

Special Somatic Receptors

Neuromast organs ('groove organ' below in Figure 10-4)receptors in skin of fishes & aquatic amphibians that detect water currents & hear soundsoccur singly, in groups, or in a linear series (e.g., lateral lines)may also be modified to detect electricity (ampullae of Lorenzini)

Shark lateral line system

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The ampullae of Lorenzini are small vesicles thatform part of an extensive subcutaneous sensorynetwork system. These vesicles are found around thehead of the shark. They detect weak magnetic fieldsproduced by other fish at short ranges. This enables ashark to locate prey buried in the sand or to orient tonearby movement. Each ampulla is a bundle ofsensory cells innervated by several nerve fibers. These fibers are enclosed in a jelly-filled tubule thathas a direct opening to the surface through a pore. These pores on the head of the shark are visible to thenaked eye, and appear as dark spots.

Neuromast organs have 2 types of cells:hair cells (receptor cells) - each hair cell has several short cilia & kinocilia that project into fluidor a cupula (displacement of cupula & cilia generates nervous impulses)supporting cells

Membranous labyrinthexhibited by all vertebratesfluid-filled & embedded in skull lateral to hindbrainLabyrinth usually consists of 3 semicircular canals, a utriculus, & a sacculus

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Semicircular canals:

Hagfish - have only one (posterior)Lamprey - have 2 (anterior & posterior)Other vertebrates - have 3 (anterior, posterior, & horizontal)

Functions of the labyrinth:

1 - Equilibrium

Dynamic equilibrium - when head moves, inertia causes a slight relative movement of fluid in at leastone semicircular canal ---> deflects cupula (in ampulla) ---> nervous impulsesStatic equilibrium - maculae (in sacculus & utriculus) tilt when head moves ---> nervous impulses

2 - Hearing - function of ORGAN OF CORTI located in lagena (enlargement of sacculus); lagena tends tobe longer in terrestrial vertebrates &, in most mammals, its coiled into the cochlea. The organ of Corticontains a specialized strip of neuromasts connected to the nervous system via the auditory nerve.

1-Inner hair cell, 2-Outer hair cells, 3-Tunnel of Corti, 4-Basilar membrane, 5-Reticular lamina,6-Tectorial membrane, 7-Deiters' cells, 8-Space of Nuel, 9-Hensen's cells, & 10-Inner spiral sulcus

[Drawing by Stephan Blatrix, from "Promenade around the cochlea" by R Pujol, S. Blatrix, T. Pujol and V. Reclar-Enjalbert, CRIC,University Montpellier 1 - INSERM. URL: http://www.iurc.montp.inserm.fr/cric/audition/english/corti/fcorti.htm]

Hearing:

Outer ear of tetrapods:Amphibians & most reptiles - eardrum (tympanic membrane) is on surface of the head

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Crocodilians, birds, & mammals - eardrum is deeper in the skull at the end of an air-filledpassageway called the outer ear canal (or external auditory meatus)Mammals - pinna collects & directs sound waves

Middle ear of tetrapods - cavity plus ossicle(s):Amphibians, reptiles, & birds - single middle ear ossicle (columella or stapes)Mammals - 3 middle ear ossicles (malleus, incus, & stapes)

Inner ear = labyrinth, including lagena (or cochlea)

How do pressure or sound waves become sound?

(Also: check out http://www.iurc.montp.inserm.fr/cric/audition/english/ear/fear.htm &

http://www.blackwellscience.com/matthews/ear.html)

Pit receptors of reptiles = infrared receptors:

1 - Labial pits

found in pythons (Family Boidae); nerve endings lie at the bottom of several recessed labial pitspermit detection of a mouse about 15 cm away

2 - Loreal pits

also called facial pits; can detect temperature changes of as little as0.001 degree C & so can detect prey several feet awaypresent in snakes in the family Crotalidae (North Americanrattlesnakes, copperheads, & water moccasins), also called the pitvipers

Light receptors (or photoreceptors) - vertebrates can perceive only a narrow band of electromagneticradiation between about 350 & 760 nm; 2 types include the epiphysis (already described) & the eye

Structure of a vertebrate eye:

Accommodation is the process of focusing light on the retina & this can occur in several different ways:

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Lamprey - contraction of corneal muscle pulls cornea against the lens & moves the lensTeleosts (bony fish) - retractor muscle attached to lens (rectractor lentis muscle) moves lensposteriorlyAmphibians & cartilaginous fish - protractor muscle attached to lens pulls the lens forward for nearvisionSnakes - increased pressure in the vitreous humor generated by muscles near the iris pushes the lensforwardMost reptiles, birds, & mammals - curvature of lens is altered by ciliary (annular) muscles

Special visceral receptors - olfactory (smell) & gustatory (taste):

Olfaction - involves receptors located in nasal passages; olfactory epithelium contains basal cells(replacement cells), supporting cells (secrete mucus), & olfactory receptor cellsvomeronasal organs - only in tetrapods but absent in most turtles, crocodiles, birds, some bats,primates, & aquatic mammals:

amphibians - recessed area off the main nasal cavityreptiles - separate pit to which tongue & oral membranes deliver chemicalsmammals:

isolated area of olfactory membrane within nasal cavity that is connected to mouth via anasopalatine ductwell-developed in monotremes, marsupials, insectivores, & many carnivores

function of vomeronasal organs - may be especially important in detecting conspecific odors,but also useful in prey detection

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Gustation (taste) - taste buds, like olfactory receptors, detect chemical stimuliTaste buds

consist of supportive cells & taste cellsdistribution:

Fish - widely distributed in roof, walls, & floor of pharynx;bottom feeders & scavengers (catfish & carp) have tastebuds distributed over entire surface of head & body,especially on the barbels (whiskers)Tetrapods - taste buds restricted to tongue, posterior palate, & oral pharynx

General Somatic Receptors - come in two categories: cutaneous receptors & proprioceptors

cutaneous receptors (for touch, pressure, pain, & temperature)naked endings - in skin of all vertebrates; stimulated by contactencapsulated endings - present in tetrapods; nerve endings wrapped in a connective tissuecapsule

Herbst corpuscles - on beak, tongue, & palate of water birdsend bulbs & Ruffini corpuscles - thermal receptors in mammalsPacinian corpuscles - touch & pressure receptors

proprioceptors - located in skeletal muscles, joints, and tendons & provide information about bodyposition

General Visceral Receptors:

mostly naked endings in mucosa of the tubes, vessels, & organs of the body, in cardiac muscle,& insmooth muscle; chiefly stretch & chemoreceptorssome functions of general visceral receptors:

monitor oxygen & carbon dioxide content of bloodmonitor blood pressure (baroreceptors in Figure 1 below)monitor concentration of solutes in blood

similar among all vertebrates

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Useful links:

Comparative Anatomy of the Chordate Nervous System

Comparative Anatomy Topic 19: The Nervous System

Coordination and Integration

Evolution of the Amniote Brain

Sensory Mechanisms

The Evolution of the Vertebrate Brain

Back to BIO 342 Syllabus

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